Cast of non-existing Combination wrongly return the item with lowest value

The cast of non-existing Combination `value1 | value2` to `TestFlagEnumWithoutCombination` wrongly returns the item with the lowest internal value (i.e. `value1`).

This is totally misleading and error-prone, moreover is not consistent with .net enums' behavior.

I think it should probably return a special runtime object (similarly to the "collection" of enum values) or just throw an exception.

```
using Ardalis.SmartEnum;

{
var value1and2_smartEnum = (TestSmartFlagEnum)(TestSmartFlagEnum.Value1 | TestSmartFlagEnum.Value2);
var value1and2_dotnetEnum = TestFlagEnum.Value1 | TestFlagEnum.Value2;

Console.WriteLine($"{value1and2_smartEnum} == {value1and2_dotnetEnum}");
// prints Value1and2 == Value1and2
}

{
var value1and2_smartEnum = (TestSmartFlagEnumWithoutCombination)(TestSmartFlagEnumWithoutCombination.Value1 | TestSmartFlagEnumWithoutCombination.Value2);
var value1and2_dotnetEnum = TestFlagEnumWithoutCombination.Value1 | TestFlagEnumWithoutCombination.Value2;

Console.WriteLine($"{value1and2_smartEnum} != {value1and2_dotnetEnum}");
// prints Value1 != Value1, Value2
}

[Flags]
public enum TestFlagEnum
{
Value1 = 1,
Value2 = 2,

// Combinations
Value1and2 = 3
}

public class TestSmartFlagEnum : SmartFlagEnum
{
public static readonly TestSmartFlagEnum Value1 = new(nameof(Value1), 1);
public static readonly TestSmartFlagEnum Value2 = new(nameof(Value2), 2);

// Combinations
public static readonly TestSmartFlagEnum Value1and2 = new(nameof(Value1and2), 3);

public TestSmartFlagEnum(string name, int value) : base(name, value) { }
}

[Flags]
public enum TestFlagEnumWithoutCombination
{
Value1 = 1,
Value2 = 2
}

public class TestSmartFlagEnumWithoutCombination : SmartFlagEnum
{
public static readonly TestSmartFlagEnumWithoutCombination Value1 = new(nameof(Value1), 1);
public static readonly TestSmartFlagEnumWithoutCombination Value2 = new(nameof(Value2), 2);

public TestSmartFlagEnumWithoutCombination(string name, int value) : base(name, value) { }
}
```

Start at the SmartFlagEnum cast behavior shown in the issue and reproduce the non-existing Value1 | Value2 case for TestSmartFlagEnumWithoutCombination. Inspect the existing SmartFlagEnum implementation and tests, then define and cover behavior that does not return the lowest-valued item for an unrepresented combination.

Written by the indexing model from the issue text.